AD7856ARS datasheet(30/32 Pages) AD

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AD7856

–30–

REV. A

Avoid running digital lines under the device as these will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7856 to avoid noise coupling. The power

supply lines to the AD7856 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply line. Fast switching signals like

clocks should be shielded with digital ground to avoid radiating

noise to other sections of the board, and clock signals should

never be run near the analog inputs. Avoid crossover of digital

and analog signals. Traces on opposite sides of the board should

run at right angles to each other. This will reduce the effects of

feedthrough through the board. A microstrip technique is by far

the best, but is not always possible with a double-sided board.

In this technique, the component side of the board is dedicated

to ground planes while signals are placed on the solder side.

Good decoupling is also important. All analog supplies should

be decoupled with 10

µF tantalum in parallel with 0.1 µF ca-

pacitors to AGND. All digital supplies should have a 0.1

µF

disc ceramic capacitor to AGND. To achieve the best from

these decoupling components, they must be placed as close as

possible to the device, ideally right up against the device. In

systems where a common supply voltage is used to drive both

the AVDD and DVDD of the AD7856, it is recommended that

the system’s AVDD supply be used. In this case there should be a

Ω resistor between the AV

DD pin and DVDD pin. This supply

should have the recommended analog supply decoupling capaci-

tors between the AVDD pin of the AD7856 and AGND and the

recommended digital supply decoupling capacitor between the

DVDD pin of the AD7856 and DGND.

Evaluating the AD7856 Performance

The recommended layout for the AD7856 is outlined in the

evaluation board for the AD7856. The evaluation board pack-

age includes a fully assembled and tested evaluation board,

documentation and software for controlling the board from the

PC via the EVAL-CONTROL BOARD. The EVAL-CON-

TROL BOARD can be used in conjunction with the AD7856

Evaluation board, as well as many other Analog Devices evalua-

tion boards ending in the CB designator, to demonstrate/evalu-

ate the ac and dc performance of the AD7856.

The software allows the user to perform ac (fast Fourier trans-

form) and dc (histogram of codes) tests on the AD7856. It also

gives full access to all the AD7856 on-chip registers allowing for

various calibration and power-down options to be programmed.

AD785x Family

12 bits, 200 kSPS, 3.0 V to 5.5 V:

AD7853 – Single-Channel Serial

AD7854 – Single-Channel Parallel

AD7858 – 8-Channel Serial

AD7859 – 8-Channel Parallel

14 bits, 333 kSPS, 4.75 V to 5.25 V:

AD7851 – Single-Channel Serial

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